Actress Jamie-Lynn Sigler surprised fans in 2016 when she shared that she has multiple sclerosis, a chronic and usually progressive autoimmune disease that damages the sheaths of the nerve cells in a person’s brain and spinal cord. People with multiple sclerosis, also known as MS, often don’t have any external signs that they’re sick—even though the symptoms, like pain and fatigue, can be debilitating—and now Sigler is opening up about how challenging that can be.

“I don’t want to say it frustrates me when people say, ‘You don’t look sick,’ because I agree, and I understand—and I don’t feel sick, sitting here in front of you,” she tells Yahoo Beauty in a new interview.

But Sigler says she’s had to make adjustments in her life due to MS, including using walking sticks when she took a recent trip to Israel. (Walking sticks and canes can help provide additional support and balance for people with MS.) “I think you can wind up feeling, like, that’s not beautiful. It’s not cute to walk with these sticks,” she says. “I talked a lot about it with my husband before I left and he was, like, ‘But Jamie, this is what’s going to allow you to go and do what you want to do, so it’s either sit it out and be resentful or take these freaking sticks and do it.’”

So, she did it. Sigler says it felt “weird” to use the walking sticks, but she made it work. “I could feel people looking at me and thinking I don’t look like somebody that needs these,” she says. “But I do, so it made me think a lot about erasing that stigma for people, and not being ashamed.” She ended up posting this picture of herself on Instagram last week with her walking sticks and received a lot of kudos from fellow MS sufferers:

Sigler says the positive feedback meant a lot to her. “We will fight as hard as we can and hold walls and people before we will get a device, because then it feels like you’re losing your battle,” she says. “It’s a process, still, for me.”

The actress says she has also undergone hypnotherapy to help her come to terms with her disease. “I wanted to go in the hopes of learning how to manage it better emotionally, because I was dealing with feeling embarrassed and ashamed,” she says. “I was always trying to hide it, and it was just really detrimental to my health and well-being and my emotional health.”

Sigler has previously spoken about struggling to accept her diagnosis. When she originally revealed to People that she has MS, she also said that she had been silently battling it for 15 years. “I wasn’t ready until now,” she said at the time. “You’d think that after all these years, somebody would be settled with something like this, but it’s still hard to accept.”

It’s not uncommon for people who struggle with an illness like MS to have no outward signs.

Sigler's heartbreaking revelations shine a light on a common illness: More than 2.3 million people around the world are affected by MS, according to the National MS Society. However, the organization notes, the Centers for Disease Control and Prevention does not require U.S. doctors to report new cases, and because symptoms can be completely invisible, there is no reliable data on how many people have the disease in the United States.

“Brain disorders tend to be less visible to the general public and even when visible, may be misinterpreted,” Farrah J. Mateen, M.D., Ph.D., a board-certified neurologist with the Multiple Sclerosis & Neuroimmunology Clinic at Massachusetts General Hospital, tells SELF. “We often don’t think of a young woman as having a chronic, potentially disabling disease when she appears well.”

Multiple sclerosis presents in different ways in different people, says Mateen—some people have only a few symptoms and then go back to feeling healthy, while other people have many symptoms that can be persistent. “It is often difficult for a person who is newly diagnosed with MS to know if his or her disease course will be benign without a lot of symptoms or rather more aggressive with many symptoms,” she says.

But MS can cause daily challenges for many people, even if the symptoms are not visible.

“Invisible symptoms such as pain, fatigue, cognitive problems, numbness, and tingling, and others may be very apparent to the person living with MS, but difficult for the person to explain and for others to truly understand or appreciate,” Kathy Costello, vice president of healthcare access at the National MS Society, and a nurse practitioner who sees MS patients at Johns Hopkins MS Center, tells SELF. “Many people with MS have little or no obvious problems, and on first glance, coworkers, friends, and even some family might not know that the person has been diagnosed with the disease.” However, she stresses, that doesn’t mean the patient isn’t suffering.

People can also suffer from terrible episodes that doctors call MS "attacks," aka flare-ups of symptoms, but those can happen months or even years apart, Mateen says. Attacks can vary between things like sensory symptoms (burning sensations and numbness), fatigue, or brain changes, and patients often don’t know when they’ll suffer an attack—but they may seem perfectly well on the outside in between those flares. “The unpredictability of MS can still understandably be a major burden to patients,” Mateen says.

When symptoms are not obvious to others, people with MS can feel that their struggles are not appreciated, Robert Charlson, M.D., assistant professor of neurology and psychiatry at NYU Langone's Multiple Sclerosis Comprehensive Care Center, tells SELF. "In a sense, patients whose struggles involve primarily severe fatigue, pain, and depression do not always receive the validation for the suffering their illness has imposed," he says. It can also make it hard to explain why they can't participate in things like family events and activities they love, as well as why they might need to miss work here and there.

Another common MS struggle is getting the proper diagnosis in the first place.

“A lot of times, the very first phases of a disease like multiple sclerosis are very vague, like numbness, tingling, and loss of vision,” Amit Sachdev, M.D., an assistant professor and director of the Division of Neuromuscular Medicine at Michigan State University, tells SELF. MS is usually ultimately found with an MRI, but Sachdev says it takes work and testing on the part of a patient and doctor to get to that point. People may also be reluctant to talk about some symptoms they experience like depression, anxiety, cognitive impairment, or fatigue, says Charlson, but these can also be symptoms of MS.

Once patients receive a diagnosis, Costello recommends that people with MS be knowledgeable about their disease and its symptoms so they can be their own health care advocates. This is the first step to creating an MS management plan, Costello says. There are different medications to help manage MS, and Mateen says choosing the right one can be “crucial.”

MS is one of many invisible illnesses, and people with other unseen conditions often share similar struggles.

For example, people with rheumatoid arthritis, chronic fatigue syndrome, depression, and diabetes often have no external signs, despite suffering health problems—and it can be difficult to make others understand. Women’s health expert Jennifer Wider, M.D., tells SELF that there are “definitely challenges” for people living with illnesses and conditions that other people can't see. “It's much easier to explain why you may have limitations if you have a [visible injury],” she says. “But for people living with these illnesses that we can't see, facing judgment and skepticism can be frustrating, anxiety-provoking, and depressing.”

Unfortunately, it’s difficult to control how strangers behave, but Sachdev says that people with an invisible illness can help influence how their loved ones react to their struggles. In those cases, it can be helpful for someone with an invisible illness to clearly explain that they need love, support, and help, he says.

But even when someone with an invisible illness has understanding loved ones, they may have a hard time wrapping their head around their diagnosis, as Sigler mentioned.

“It can be really hard to understand why your body isn’t working right even though you can’t see that something is obviously wrong,” Sachdev says. “That’s a big barrier for patients.”

Above all, experts say it’s important for people suffering from an invisible illness like MS to get the love and support of their family and friends—and to do have them do their own research about the disease. “Encouraging family members to become more knowledgeable about MS and MS symptoms is important in improving understanding and appreciation for the challenges the person with MS is facing—often on a daily basis,” Costello says.